There are a variety of processes which utilize alkali-based chemicals such as sodium hydroxide in the pulping, bleaching or oxidation of wood materials. These processes include chemical and semi-chemical methods for breaking down wood chips or other wood-based starting materials into wood fiber for the production of paper, cardboard and similar cellulose-based products. Other processes which use such alkali-based chemicals include the oxidizing and bleaching of wood pulp for paper production.
In a typical chemical-based wood pulping process, such as the kraft process, wood chips are treated with an aqueous solution of mainly sodium hydroxide (caustic soda) to separate out lignin and other organic constituents which bind the cellulose fibers together in order that the wood can be broken down into individual fibers for various uses such as paper making. In the kraft process this solution also contains sodium sulfide. The sodium hydroxide reacts and combines chemically with lignin forming an organic-based solution referred to as black liquor or spent liquor. The black liquor is separated from the fiber and burned in a recovery boiler to recover heat from the organics. In the process of burning, the black liquor is converted into smelt, a molten phase in which the sodium-organic complex has been converted to sodium carbonate. In the kraft process sodium sulfide is also formed. In order to regenerate sodium for reuse in the pulping process, sodium carbonate must be converted back to sodium hydroxide or "recausticized". The molten smelt is typically dispersed with steam as it is poured into an aqueous solution, such as recycled dilute white liquor, weak wash or water, in which it dissolves to form a sodium carbonate solution referred to as green liquor due to the dark green appearance caused by the presence of an insoluble residue known as dregs. In some operations, such as in the soda process, the smelt is cooled and solidified prior to dissolution. The green liquor is sent to a causticizer where sodium carbonate is converted back to sodium hydroxide, thus producing "white liquor" for reuse in the wood pulping process. This process, known as causticization, is accomplished by the reaction of sodium carbonate with calcium hydroxide, also known as hydrated or slaked lime, in the green liquor. In the process of regenerating sodium hydroxide, the calcium hydroxide is converted to calcium carbonate, as a precipitate (also known as lime mud), which is then converted back to calcium hydroxide in a separate lime recovery circuit, also known as a lime recovery cycle, so that it can be reused in the causticizer again. In the lime recovery circuit calcium carbonate is burned in a kiln to drive off carbon dioxide as a gas, converting the calcium carbonate to calcium oxide, which is then hydrated with water in the green liquor to reform calcium hydroxide which can be reused in the causticization step.
An alternative method of recausticization which does not require the use of lime and the associated lime recovery process was developed in the 1970's by Jan Janson, a researcher in Finland (U.S. Pat. No. 4,116,759). Janson proposed that sodium carbonate in the smelt could be causticized automatically ("autocausticized") in the recovery boiler by the addition of borate to the wood pulping circuit, thus eliminating the need for subsequent recausticization by calcium hydroxide and the accompanying lime recovery circuit.
The chemical reactions proposed by Janson for the autocausticization process were:
(1) Cooking or bleaching (delignification): EQU Na.sub.2 HBO.sub.3 LignOH{character pullout}LignONa+NaH.sub.2 BO.sub.3 PA1 (2) Combustion: EQU 2 LignONa+x.O.sub.2.fwdarw.Na.sub.2 CO.sub.3 +y.CO.sub.2 +zH.sub.2 O PA1 (3) Autocausticization: EQU 2 NaH.sub.2 BO.sub.3 +Na.sub.2 CO.sub.3.fwdarw.2 Na.sub.2 HBO.sub.3 +CO.sub.2 +H.sub.2 O
In autocausticizing, sodium metaborate acts like a catalyst, in that it will react with sodium carbonate in the smelt to produce a more basic disodium borate and carbon dioxide. When the disodium borate is dissolved in water, it is hydrolyzed to regenerate sodium hydroxide and the original sodium metaborate, hence eliminating the need for lime and the lime kiln and associated lime recovery cycle.
In a typical kraft process, sodium hydroxide is recovered for reuse in the process using the traditional lime recausticization methods described above. Autocausticization offers several potential benefits over recausticization with lime. These include elimination of the capital costs associated with the lime recovery circuit, reduction of energy costs by elimination of the need to burn the calcium carbonate to release carbon dioxide and elimination of other operating costs associated with the lime recovery circuit. Alternatively, in some operations, where sodium is not being recovered and reused in the process, autocausticization offers significant potential cost savings due to reduced chemical requirements, since borate is not used up in the process, but is instead returned to the start of the process for reuse along with the regenerated sodium hydroxide. However, Janson teaches in the '759 patent that it is essential to keep the sodium to boron molar ratio equal to or less than 2 (Na/B.ltoreq.2) in order to ensure complete causticization.
Large scale trials (Janson, Jan and Bengt Arhippainen,"Mill Scale Development of the Borate-Based Kraft Pulping Process", International Conference on Recovery of Pulping Chemicals, Vancouver, British Columbia, Canada, Sep. 22-25, 1981) were conducted in the early 1980's to investigate the commercial applicability of autocausticization using borate. However, operating difficulties were encountered and the process was never adopted on a commercial basis. Such difficulties are largely related to changes in the physical properties of the black liquor due to the presence of high levels of borate, such as large increases in the dissolved solids content and viscosity, leading to difficulties with spraying and droplet size in the recovery boiler, reduced evaporation rate and the transporting of the liquor from the digestor to the recovery boiler. Also, a reduction in the heating value of the black liquor may require the addition of supplemental fuel in the recovery boiler.
Despite the potential benefits offered by autocausticization, it has not been adopted commercially in view of the problems associated with the process. It is an object of this invention to provide an improved causticization process which will provide some of the significant benefits of autocausticization, while minimizing the difficulties associated with it.